1. Field of the Invention:
The present invention is to a nuclear fuel element for use in nuclear reactors which fuel element is resistant to pellet-clad interaction.
2. Description of the Prior Art:
Nuclear reactor systems for producing electrical energy provide heat for steam production, with the steam produced used for electrical energy production. In a nuclear reactor, nuclear fuel rods are present which preferably contain an enriched nuclear fuel such as pellets of uranium dioxide enriched in the U-235 isotope. Such nuclear fuel rods are metallic tubular shells, or cladding, which are hermetically sealed at their ends and contain the fuel pellets. Such cladding must maintain its integrity so as to prevent any leakage into or out of the shell of a pressurized water nuclear reactor or a boiling water nuclear reactor.
Nuclear fuel cladding is normally formed from zirconium or a zirconium alloy. During operation of a nuclear reactor, fissile materials are released from the fuel pellets. These released materials, which include volatile materials, cause a problem of stress corrosion and possible failure of the metallic tubular cladding. This phenomenon is generally described as "pellet-clad interaction" (PCI). The chemical reaction of the metallic tubing with high energy fissile materials such as iodine, cesium, cadmium, or other elements, coupled with cladding operating stresses can produce stress corrosion cracking of the metallic cladding or tubing and resultant penetration of the wall of the tube. In order to prevent pellet-clad interaction, utilities often operate a nuclear reactor plant in such a conservative manner as to lose about three to five percent in plant availability.
Attempts have been made to prevent pellet-clad interaction, such as by coating the inner surface of the tubing wall with various protective coatings, or coextruding a pure zirconium barrier on the inner portion of a zirconium alloy tubular wall. In U.S. Pat. No. 3,925,151, for example, it is suggested to dispose in the tubular cladding, between the cladding wall and the fuel pellets, a metal liner, such as a liner of zirconium or an alloy of zirconium. A layer of a high lubricity material such as graphite or molybdenum disulfide is disposed between the cladding and the liner, which acts as an interface between the liner and the cladding and reduces localized stresses on the claddng due to fuel expansion and cracking of the fuel. The use of such a high-purity, low-oxygen zirconium liner in a cladding, while useful, results in high costs associated with such fuel elements.
It has also been proposed to provide a zirconium cladding that has a graphite layer on the inner surface thereof between the cladding wall and the fuel. Such efforts have been addressed to preventing pellet-clad interaction in natural uranium CANDU reactors In U.S. Pat. No. 4,146,431, for example, a nuclear fuel element containing non-enriched uranium is described wherein a non-bonding lining layer is interposed between the pellets and the cladding, which layer is selected from graphite, siloxane and silicon. The non-bonding layer may be applied to the interior surface of the cladding or the same may be applied to the surface of the non-enriched uranium dioxide fuel pellets.
U.S. Pat. No. 4,146,431 also refers to U.S. Pat. No. 3,119,747 which discloses use of a high temperature lubricant such as graphite between the fuel body and the fuel rod sheath, in connection with an elongated fuel body having a gradual end taper. Alternatively or additionally, a thin coating, for example less than 0.005 inch thick, of a high temperature stable lubricant such as molybdenum disulfide can be applied to the outer surface of the fuel body and/or the inner surface of the container.
Unfortunately, such graphite layers as taught in U.S. Pat. Nos. 4,146,431 and 3,119,747, although extremely effective, lose their effectiveness with burnup and are essentially ineffective above 10,000 to 15,000 megawatt days/metric tons of uranium (MWD/MTU) of fuel burnup. Fission products from the fuel react chemically with the graphite and cause the graphite to lose its lubricating properties. The graphite is attractive, however, in that it is relatively inexpensive to put a graphite coating on the inner surface of the cladding.
The coating of nuclear fuel pellets with materials other than lubricants has also been proposed for various reasons. In U.S. Pat. No. 3,427,222, for example, assigned to the assignee of the present invention, fuel pellets are described havng a fusion-bonded coating thereon which is composed of a boron-containing material that functions as a burnable absorber.
It is an object of the present invention to provide a nuclear fuel element containing enriched uranium dioxide fuel which is immune to pellet-clad interaction fuel failures.
It is another object of the present invention to provide a nuclear fuel element containing enriched uranium dioxide fuel in which both conditions required for pellet-clad interaction, the damaging chemical species and high-clad stresses, are reduced so as to provide an improved life of the nuclear fuel element.